arXiv subjects

Jordi Pera

Publications and source records attributed to Jordi Pera.

6 recordsLinked to original sources

arXiv2026

Mass-imbalanced SU(N) Fermi gases

We report a fully analytical description of zero-temperature itinerant ferromagnetism in repulsive SU(N) Fermi gases with arbitrary mass imbalance among components. Using perturbation theory in the gas parameter x = kFa0, with kF the Fermi momentum and a0 the s-wave scattering length, we derive the second-order energy for arbitrary spin polarization and arbitrary mass ratio. Our main result is a closed analytic expression for the beyond-mean-field correction in mixtures with unequal masses. This analytical result extends the theory of dilute Fermi gases beyond the mass-balanced case and provides a compact equation of state for multicomponent mixtures. We show that mass imbalance breaks the paramagnetic symmetry already in the non-interacting limit, favors the occu pation of heavier components, and lowers the interaction strength required to reach a fully polarized state. For S = 1/2, the system evolves continuously from a mass-induced partially polarized state to full ferromagnetism. For larger spins, distinct mass distributions generate qualitatively differ ent polarization paths, including smooth and discontinuous sequences. Our results identify mass imbalance as a powerful control tool for magnetic ordering in ultracold Fermi mixtures.

cond-mat.quant-gas↗

arXiv2024

Equation of state of Bose gases beyond the universal regime

The equation of state of dilute Bose gases, in which the energy only depends on the $s$-wave scattering length, is rather unknown beyond the universal limit. We have carried out a bunch of diffusion Monte Carlo calculations up to gas parameters of $10^{-2}$ to explore how the departure from the universality emerges. Using different model potentials, we calculate the energies of the gas in an exact way, within some statistical noise, and report the results as a function of the three relevant scattering parameters: the $s$-wave scattering length $a_0$, the $s$-wave effective range $r_0$, and the $p$-wave scattering length $a_1$. If the effective range is not large we observe universality in terms of $a_0$ and $r_0$ up to gas parameters of $10^{-2}$. If $r_0$ grows the regime of universality in these two parameters is reduced and effects of $a_1$ start to be observed. In the $(a_0,r_0)$ universal regime we propose an analytical law that reproduces fairly well the exact energies.

cond-mat.quant-gas↗

arXiv2024

Interaction effects on the itinerant ferromagnetism phase transition

Itinerant ferromagnetism is one of the most studied quantum phase transitions, the transition point and the nature of this phase transition being widely discussed. In dilute Fermi liquids, this analysis has been carried out up to second-order in the gas parameter, where the results for any spin degeneracy are universal in terms of only the s-wave scattering length $a_0$. We extend this analysis to third-order where energies depend, not only on $a_0$, but also on the s-wave effective range $r_0$ and the p-wave scattering length $a_1$. The introduction in the theory of these new parameters changes the transition point, with respect to the second-order estimation, and also can modify the nature of the phase transition itself. We analyze these interaction effects on the phase transition for different spin values. The emerging phase diagram shows that the type of ferromagnetic transition changes dramatically as a function of $r_0$ and $a_1$ and, importantly, that this classification is not solely determined by the spin value, as happens at second order.

cond-mat.quant-gas↗

arXiv2023

Low-energy scattering parameters: A theoretical derivation of the effective range and scattering length for arbitrary angular momentum

The most important parameters in the study of low-energy scattering are the s-wave and p-wave scattering lengths and the s-wave effective range. We solve the scattering problem and find two useful formulas for the scattering length and the effective range for any angular momentum, as long as the Wigner threshold law holds. Using that formalism, we obtain a set of useful formulas for the angular-momentum scattering parameters of four different model potentials: hard-sphere, soft-sphere, spherical well, and well-barrier potentials. The behavior of the scattering parameters close to Feshbach resonances is also analyzed. Our derivations can be useful as hands-on activities for learning scattering theory.

quant-ph↗

arXiv2022

Itinerant ferromagnetism in dilute SU(N) Fermi gases

We present exact analytic results for the energy of a SU(N) repulsive Fermi gas as a function of the spin-channel occupation at second order in the gas parameter. This is an extension of an old result that now incorporates the degree of polarization of the system. Therefore, the magnetic properties of the gas can be obtained, free from numerical uncertainties. For spin 1/2 we find that second-order corrections change the itinerant ferromagnetic transition from continuous to first-order. Instead, for spin larger than 1/2 the phase transition is always of first-order type. The transition critical density reduces when the spin increases, making the phase transition more accessible to experiments with ultracold dilute Fermi gases. Estimations for Fermi gases of Yb and Sr with spin 5/2 and 9/2, respectively, are reported.

cond-mat.quant-gas↗

arXiv2022

Beyond universality in repulsive SU(N) Fermi gases

Itinerant ferromagnetism in dilute Fermi gases is predicted to emerge at values of the gas parameter where second-order perturbation theory is not accurate enough to properly describe the system. We have revisited perturbation theory for SU(N) fermions and derived its generalization up to third order both in terms of the gas parameter and the polarization. Our results agree satisfactorily with quantum Monte Carlo results for hard-sphere and soft-sphere potentials for $S = 1/2$. Although the nature of the phase transition depends on the interaction potential, we find that for a hard-sphere potential a phase transition is guaranteed to occur. While for $S= 1/2$ we observe a quasi-continuous transition, for spins $3/2$ and $5/2$, a first-order phase transition is found. For larger spins, a double transition (combination of continuous and discontinuous) occurs. The critical density reduces drastically when the spin increases, making the phase transition more accessible to experiments with ultracold dilute Fermi gases. Estimations for Fermi gases of Yb and Sr with spin $5/2$ and $9/2$, respectively, are reported.

cond-mat.quant-gas↗